Nitric oxide is involved in regulation of a variety of biological functions and serves as a messenger for neurons in the brain and other parts of the body. Nitric oxide activates a soluble form of guanylate cyclase in a number of tissues. Recently, we and others have shown by molecular cloning that a number of guanylate cyclases are present in human and rat retina. In this study we have shown the presence of nitric oxide synthase mRNA in human retina. We have used reverse - transcriptase polymerase chain reactionm (RT-PCR) and standard cloning techniques to detect the constitutive form of NOS (c-NOS) expressed in human retina. The degenerate primers using inosine were designed for PCR reaction from the conserved region of the published sequence of the cDNAs of c-NOS and PCR amplification resulted in an amplification product of about 1.4kb. The PCR product was purified and subcloned and sequenced. The sequence comparison has shown almost 100% identity with the published c-NOS cDNA sequence from human brain. This product is being used both to screen the cDNA library from human retina and for the Northern blot analysis of human retinal mRNA. The expression of such form in human retina should help in understanding the retinal degeneration in certain pathological conditions, such as retinitis pigmentosa.